Signal conversion apparatus converts an input signal in a first form to an output signal in a second form. The first and second forms may each be either analog or digital. Accordingly, the conversion apparatus may provide analog-to-digital conversion and vice versa, digital-to-digital conversion, or analog-to-analog conversion. This invention relates to apparatus which provides any of these conversions except for the last. In other words, the form of the input and/or output signal is digital.
In many telecommunications applications, analog signals are quantized into discrete digital signal levels. The analog signal may represent virtually any information such as voice, data, facsimile, video and the like. Quantization involves assigning the amplitude of an analog signal sample to one of a number of discrete output levels. This process inherently produces "rounding-off" or quantization errors which result from the association of a continuum of amplitude values between some prescribed limits with a far smaller number of discrete digital signal levels.
The above errors are particularly significant within communications networks in which digital loss pads are routinely inserted within circuits to attenuate echoes. Such loss pads, of course, also attenuate the information signal. However, as the level of the information signal far exceeds that of the objectionable echo and the minimally acceptable information signal levels, the use of such pads provides an inexpensive alternative to the use of echo cancellers and suppressors in circuits less than some predefined length. This circuit length varies but is typically less than 1000 miles.
The circuit loss inserted will vary with the particular circuit application and each circuit is engineered to conform to a particular loss plan. Moreover, when circuits constructed in accordance with different loss plans are interconnected together, it is often necessary to insert some predetermined amount of gain into the information signal.
The use of loss or gain pads produces an additional quantization error for the information signal inasmuch as the quantized signal, when altered by the predefined gain or loss, may not exactly correspond to one of the discrete digital signal levels. Hence, another rounding-off error is introduced. This additional quantization error, while introducing additional distortion in voice applications, can produce a particularly objectionable level of errors in data, video and facsimile applications. This is particularly true in applications where a customer modem converts digital information into an analog signal for transmission through a communications path which interconnects the customer to the rest of the communications network. In such applications, the attenuation of the information and echo signals limits the ability of an echo suppressor or canceller in the modem to correctly identify these signals and the transmission error rate dramatically increases. Furthermore, inasmuch as the additional quantization error varies and is unpredictable, present attempts to solve this problem have not been successful.